Sandia National Laboratories is a multimission laboratory
managed and operated by National Technology & Engineering
Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell
International Inc., for the U.S. Department of Energy’s National
Nuclear Security Administration under contract DE-NA0003525.
Solid State Separator Development for Sodium Batteries
Motivation: Sodium batteries hold significant potential as safe,
low-cost, long cycle-life batteries. They are among of the DOE
Office of Electricity’s (OE) battery research priorities, part of
the OE mission to ensure a resilient, reliable electrical grid.
Effective zero-crossover solid state separators remain a critical
part of sodium battery performance and represent a potential point
of failure in these batteries
Objective: We aim to create zero-crossover solid state
separators that enable low temperature (10-3 S/cm at 25oC,
Ceramatec, Inc.) and established chemical compatibility,
NaSICONceramics remain attractive separators for Na-batteries.
Key Separator Properties:• Selective, high ionic conductivity at
reduced temperature (94% density and >0.4 mS/cm at 25oC using
low cost materials.
Toward Improved NaSICON Production
• Unexpected pitting, cracking and void formation in the
ceramics, however, periodically led to battery failures!
20 µm
Pitting in the surface of a sintered, sectioned
NaSICONpellet.
• In FY20 we hypothesized these defects were related to poor
compaction on pressing the green (unfired) ceramics.
• We systematically identified that 1) introducing 2% PVB
binder, 2) controlling moisture content during processing, and 3)
increasing pressing force helped resolve this issue.
In FY19, we recognized that sodium-based clays (e.g.,
montmorillonite) showed promising ionic conductivity.
FY20 Objectives: 1) characterize pellet structure and mechanical
properties* and 2) and explore clay use as electrochemical
separators.
*Please see poster by Ryan Hill, “Mechanical and Microstructural
characterization of Montmorillonite (MMT) Sodium Ion Conductors”
for mechanical assessment of clays and clay-based composites.
Microstructure Analysis:
200 nm
Plan View
100 µm
X-Sect1” MMT Pellet
Scanning electron micrographs clearly showed clay platelet on
pellet surface and a dense, clay-packed cross-section needed for
conductivity.
Separator Evaluation: Can the low temperature conductivity of
the clay make it a useful electrochemical separator?
The water content in clay, key to its conductivity, proved too
reactive in molten sodium batteries. The relative solubility of
clay against common aqueous and polar non-aqueous solvents made it
impractical to use in traditional sodium-ion systems.
“Anode”: NaxMnO2 + Hard CarbonSolid State
Electrolyte:Na-MMT“Cathode”: NaxMnO2 + Hard Carbon Na
+
Na+
What about a solid state system?
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
0 2 4 6 8 10
Ove
rpot
entia
l (V)
Time (h)
Low current galvanostatic cycling in a symmetric pseudocapacitor
shows MMT separator will facilitate reversible charge transport
between the two symmetric MnO2-based electrodes!
Improved electrode composition and interfaces are needed to
reduce high overpotentials, though.
Programmatic Accomplishment: Spoerke, et al. “Clay based ion
conductors: A dirt cheap separator?” In preparation for submission
to J. Mater. Chem. A. (2020).
Ceramic density increased to >96% density, and conductivity
increased to 0.55 mS/cm. Key Result: High quality pellet yield was
increased, producing a critical supply of NaSICONneeded to sustain
progress in the Na-battery program.
2ZrSiO4 + Na3PO4 à Na3Zr2PSi2O12
In our approach, we employ a solid state synthesis using low
cost materials:
SEM image credit: Sara Dickens (SNL)
0.1 mA/cm2
• Continued refinement of NaSICON synthesis yielded material for
a reliable source of lab-scale separators needed for Na-battery
testing.
• Undesirable reactivity and solubility undermined the potential
of low cost, high conductivity clay separators in molten sodium and
solvent-based sodium ion batteries.
• Low current density cycling in a solid state symmetric
pseudocapacitor shows a potential pathway toward utilizing clays as
a low-cost next generation separator.
Future materials innovations built on these results promise new,
low-cost, high conductance solid state separators!
